1. Field of the Invention
This invention relates to an apparatus, method and program for creating a luminance conversion curve that is for the purpose of converting the brightness of an image.
2. Description of the Related Art
Digital image data is subjected to a variety of correction processing for the purpose of facilitating the viewing of and improving the quality of the image represented by the digital image data. A density conversion (luminance conversion, brightness conversion, tone conversion), which is one type of correction processing, generally corrects digital image data in such a manner that an image represented by the digital image data will be brightened if the image is dark and darkened if the image is bright.
The specification of Japanese Patent Application Laid-Open No. 2000-196890 (Patent Document 1) describes an image processing apparatus so adapted that if the density of the image of a subject is not within a target density range, the tone of the image is converted in such a manner that the density of the image of the subject will fall within the target density range in a state in which the densities of maximum- and minimum-density areas of the image are approximately maintained. A tone curve for the density correction is as illustrated in FIGS. 3 and 4 of the disclosed specification.
With the image processing apparatus described in Patent Document 1 (the density correction in accordance with the tone curve illustrated in FIGS. 3 and 4), the density of the image of the subject can be brought within the target density range and the densities of the maximum- and minimum-density areas of the image are not corrected. Since there is no decline in the density of highlight areas in the image and no rise in the density of shadow areas, the image can be prevented from becoming one giving a bland impression.
However, the tone curve not only stipulates the density (brightness and darkness) of the image after the correction but also is related to the saturation of the image. If the density correction is performed so as to further lower the density (raise the luminance) of the image of the subject when the density of the image of the subject is low (the luminance value is high) at the start, there are instances where the saturation of the image after the correction rises so much that the image obtained after the correction gives an impression far removed from that of the original image. There are also instances where the hue of the image after the correction differs from that of the original image.
Further, if the slope of the tone curve is extremely large, a portion of the image having a density that corresponds to a portion where the curve has a large slope undergoes a density correction that is too extreme (i.e., the image is converted to a high-contrast image).